The invention relates to a retarded or electronically braked electric motor comprising at least one field winding and a commutating armature being connected in series with the at least one field winding and being fed during operating mode by a supply voltage, and further comprising means for switching into a braking mode.
The invention further relates to an electric motor configured as an asynchronous motor comprising a squirrel cage rotor.
A retarded or electronically braked electric motor as mentioned at the outset and designed as a series motor is known from WO 91/03866.
Herein, for braking the series motor, a closed circuit disconnected from the power supply is formed via the field windings, commutating windings and the armature when switching from operating mode into braking mode. For limiting the braking current, a connection is formed between the armature and a commutating winding, this being controlled by two Zener diodes connected inversely parallel.
For switching between operating mode and braking mode, a multiple pole reversing switch is necessary that must have a specific switching sequence to avoid a destruction of the switch by the forming of a light arc when switching quickly back and forth between operating mode and braking mode.
From DE 3 636 555 A1 another series motor has become known which shall offer a self-excited braking when switching from operating mode into braking mode. To guarantee an initiation of braking, a capacitor is provided that is charged during operating mode by means of a diode. When switching into braking mode, the braking is initiated by discharging the capacitor.
Also this electric motor requires a special multiple pole switch which must be designed so as to reliably disconnect the current circuit from the supply voltage, before the braking circuit is closed via one field winding and the armature. In addition, the utilization of a capacitor is seen as a disadvantage, since only one single capacitor discharge is possible, to initiate braking of the motor. If this should not be sufficient to effect a reliable initiation of the braking mode, then also later no braking will be possible, once the capacitor is discharged.
U.S. Pat. No. 5,828,194 to Canova discloses a control circuit of a DC series motor comprising two half bridges each with a respective switch with controlled opening and closing. The first half bridge connects a pole of the field winding to the battery and the second of the half bridge connects a pole of the armature to the battery. The control circuit is designed to feed power resulting from the braking operation back into the battery. To this end the armature must be pole reversed which is affected by contactors for reversing. However, the utilization of mechanical contactors for switching between an operating and a braking mode is considered a major disadvantage.
U.S. Pat. No. 6,236,177 to Zick et al. discloses a braking and control circuit for a series motor used in an electric power tool. A triac in series with the armature and field windings is controlled by a microcontroller for controlling the motor during operating mode and during braking mode. During braking mode the armature is bypassed by a triac also controlled by the microcontroller. However, since the design utilizes fast switching electronic components high excess voltages may arise there from which may impair the reliability of the motor.
In asynchronous motors comprising squirrel cage rotors, up to now no braking device has become known.